Wave division multiplexing (WDM) optical networks are well known. A WDM channel is typically transmitted by a single mode semiconductor laser, where information to be communicated is imposed on the light by modulating the laser current or by externally modulating the light by applying a voltage to a modulator coupled to the laser source. A receiver subsequently photo-detects and converts the light into electric current either by direct or coherent detection.
Due to the rapid growth of optical networks and the need for greater capacity, significant research has focused on finding efficient multi-level optical modulation formats. Any digital modulation scheme uses a finite number of distinct signals to represent digital data. Phase-shift-keying (PSK) uses a finite number of phases; each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits, and each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thereby recovering the original data. The receiver compares the phase of the received signal to a reference signal. This expedient utilizes coherent detection and is referred to as CPSK.
Alternatively, in lieu of using the bit patterns to establish the phase of the wave, CPSK employs differential phase changes. The demodulator then determines these phase changes in lieu of the actual phase of the signal. This scheme is referred to as differential phase-shift keying (DPSK), and is easier to implement than PSK as there is no need for the demodulator to maintain the reference signal to determine the exact phase of the received signal.
BPSK (also sometimes called PRK, Phase Reversal Keying) is the simplest form of PSK. It utilizes a pair of phases separated by 180° and is known as 2-PSK.
Quaternary or quadriphase PSK, 4-PSK, or 4-QAM (QPSK) uses four points on a constellation diagram as is known in the art. The four-phase QPSK can encode two bits per symbol—twice the rate of BPSK—and experimentation has demonstrated that this may double the data rate compared to a BPSK system while maintaining the bandwidth of the signal. Alternatively, QPSK can maintain the data-rate of BPSK at half the requisite bandwidth.
Optical modulations based on four-level quadrature-phase-shift-key (QPSK) have been effectively demonstrated for both 40 Gb/s and 100 Gb/s optical transmission. In the quest for even higher spectral efficiency, eight-level 8-PSK modulation has been proposed and demonstrated experimentally.
8-QAM is another eight-level modulation format. In comparison to 8-PSK, 8-QAM is tolerant of greater noise (on the order of 1.6 dB), with identical spectral utilization.
Optical modulation formats based on 4-ary quadrature-phase-shift-key (QPSK) and 8-PSK have already been demonstrated for 100 Gb/s optical transmission are discussed in the publications by Xiang Zhou, Jianjun Yu, Mein Du, and Guodong Zhang, “2 Tb/s (20×107 Gb/s) RZ-DQPSK straight-line transmission over 1005 km of standard single mode fiber (SSMF) without Raman amplification,” OFC 2008, paper OMQ3; P. J. Winzer, G. Raybon, S. Chandrasekhar, C. D. Doerr, T. Kawanishi, T. Sakamoto, K. Higuma, “10×107 Gb/s NRZ-DQPSK transmission at 1.0 b/s/Hz over 12×100 km including 6 optical routing nodes,” Proc.OFC2007, Anaheim, Calif., 2007, PDP 24; Xiang Zhou, Jianjun Yu, Dayou Qian, Ting Wang, Guodong Zhang, and P. D. Magill, “8×114 Gb/s, 25-GHz-spaced, PolMux-RZ-8PSK transmission over 640 km of SSMF employing digital coherent detection and EDFA-only amplification,” OFC 2008, PDP1; and M. Seimetz, L. Molle, D.-D. Gross, B. Auth, R. Freund, “Coherent RZ-8PSK transmission at 30 Gb/s over 1200 km employing Homodyne detection with digital carrier phase estimation,” Proc. ECOC2007, Berlin, September 2007, paper We 8.3.4,
Rectangular 16-QAM has been shown to be a very attractive modulation format to further increase the spectral efficiency as disclosed in Joseph M. Kahn and Keang-Po Ho, “Spectral Efficiency Limits and Modulation/Detection Techniques for DWDM Systems,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 10, No. 2, March/April 2004, pp. 259-272. As shown by Kahn, rectangular 16-QAM can increase the spectral efficiency (SE) by 33% over 8-PSK while only introducing a 0.5 dB noise penalty.
A method to generate rectangular 16-QAM optical signals is proposed by Y. Mori, C. Zhang, K. Igarashi, K. Katoh, K. Kikuchi, “Unrepeated 200-km Transmission of 40-Gbit/s 16-QAM Signals using Digital Coherent Optical Receiver,” OECC 2008, 2008, PDP 4; and P. J. Winzer, A. H. Gnauck, “112 Gb/s Polarization-Multiplexed 16-QAM on a 25-GHz WDM Grid,” ECOC 2008, 2008, paper Th.3.E.5, The method uses a digital or analogue method to generate two multilevel electrical signals which are then used to drive a dual parallel Mach-Zehnder modulator (MZM). At present, the generation of high-speed multilevel electrical signals is still quite difficult and may be very expensive due to the need of broadband linear electrical power amplifiers.
Another method to also generate rectangular 16-QAM optical signals is proposed T. Sakamoto et al, ECOC '07, PD2.8, 2007. The method uses four binary electrical signals to drive a quadparallel MZM, which consists of two amplitude-asymmetric dualparallel MZMs in a parallel configuration. In addition, very accurate phase and polarization control of the two amplitude asymmetric dual-parallel MZMs are required, which may also be a potential problem for the practical application. In fact, quad-parallel is still not commercially available yet.